1. Field of the Invention
This invention relates to a method and apparatus for blending oxygen in a piston-type ventilator which permits constant and accurate supply of supplemental oxygen to the user through a closed loop pressure support ventilation system. More particularly, it is concerned with a piston ventilator used to ventilate patients having difficulty respirating, which adjusts the amount of oxygen supplied both to the piston cylinder and to the patient circuit for ensuring a satisfactory oxygen content throughout the air in the circuit inhaled by the patient.
2. Description of the Prior Art
Ventilation systems are designed to supply air at a pressure greater than atmospheric pressure to assist in breathing, and may include systems which provide supplemental oxygen which is blended with air to enrich the gas which is inhaled. Such systems are typically employed for patients with respiratory ailments wherein the oxygen-enriched air is provided by blending bottled air or less often atmospheric air with supplemental oxygen in a controlled manner for each breath.
Another system for mixing the oxygen with air or another gas in a ventilation system in predetermined proportions involves the use of separate inlets into a pressure vessel up to respective first and second pressures is described in U.S. Pat. Nos. 4,022,234 and 4,023,587. The system shown therein operates in alternating withdrawal and mixing cycles. A feedback control of the rate of flow and pressure of breathing gas to a patient by an inspiration servounit is described in U.S. Pat. No. 3,741,208. U.S. Pat. No. 5,383,449 provides for control of oxygen concentration in a breathable gas by calculation of the mole ratios and pressure in the containment vessel, and by sequentially injecting oxygen and another gas to desired pressure values. These so-called batch mixing ventilators represent one system for patient ventilation.
While such systems are very useful in hospitals and other health care facilities, smaller and more confined devices not requiring connection to pressurized air are often more appropriate for home care. Piston and bellows types of ventilators allow delivery of a predetermined volume of breathing gas at a desired pressure responsive to the initiation of inspiratory efforts by a patient. Piston based ventilators can typically be made to be more compact than bellows based ventilators, but piston ventilators typically blend pressurized air and oxygen in a high pressure blender. The resultant mixture is then drawn by a piston through a valve that reduces the pressure of the mixture. Such systems typically do not permit the use of room air and pressurized oxygen, and can result in some risk of overpressurization in the event of failure of a high pressure gas delivery valve controlling introduction of one of the breathing gas components into the high pressure blender.
Another system for blending oxygen in a ventilator is shown in International Publication No. WO 96/24402 published Aug. 15, 1996. This system is designed for mixing gases at approximately ambient atmospheric pressure, such as oxygen and air. The mixing apparatus includes a piston disposed within a pump chamber. A flow limiting inlet controls introduction of oxygen for mixing with air, and the pressure of the oxygen is limited to an acceptable maximum pressure whereby even if the oxygen valve fails, the breathing gas will not be provided at an excessive pressure. A demand valve is alternately provided for reducing the pressure of the oxygen supplied before mixing, and a pressure sensor is also provided downstream of the demand valve for detecting failure of the demand valve to shut off the supply of the oxygen to prevent overpressurization.
It would be desirable, however, to provide a piston ventilator where oxygen can be blended with gas or air where the piston causes air to be provided to the patient in a cycle which more closely approximates the patient's inhalation and expiration profile. A disadvantage of using a constant rate of piston movement within the cylinder to produce ventilation flow is that the flow is affected by changes in gas density and altitude, and thus requires the use of barometric pressure monitoring and input to control the piston movement rate. In turn, it would be desirable to monitor the flow of the gas breathed by the patient and provide oxygen blending based on the flow and feedback controls based on the flow and the volume of gas in the piston cylinder to permit use of less expensive valve controls. It is also desirable to provide oxygen blending in such a piston system where oxygen enrichment can be provided for air remaining in the ventilator system downstream from the piston system after exhalation by the patient.